Method and apparatus for discharging a superconducting magnet

ABSTRACT

Circuitry detects a quench in a superconducting magnet and discharges the superconducting magnet into a load, such as a utility system, at a substantially constant voltage. The circuitry can be an inverter, arranged between the superconducting magnet and the load, which may operate in overload mode during discharge. Discharging occurs until the amount of energy in the superconducting magnet is below a predetermined level.

INCORPORATION BY REFERENCE

The following applications are hereby incorporated by reference into thesubject application as if set forth herein in full: (1) U.S. patentapplication Ser. No. 09/240,751, entitled “Electric Utility Network WithSuperconducting Magnetic Energy Storage”, filed Jan. 29, 1999; (2) U.S.Provisional Application No. 60/117,784, entitled “Electric UtilityNetwork With Superconducting Magnetic Energy Storage”, filed Jan. 29,1999; (3) U.S. patent application Ser. No. 09/449,375, entitled “MethodAnd Apparatus For Providing Power To A Utility Network”, filed Nov. 24,1999; (4) U.S. patent application Ser. No. 09/449,436, entitled “MethodAnd Apparatus For Controlling A Phase Angle”, filed Nov. 24, 1999; (5)U.S. patent application Ser. No. 09/449,378, entitled “Capacitor BankSwitching”, filed Nov. 24, 1999; (6) U.S. Provisional Application No.09/718,672, entitled “Voltage Regulation Of A Utility Power Network”,filed Nov. 24, 1999.

BACKGROUND OF THE INVENTION

This invention relates to discharging a superconducting magnet at asubstantially constant voltage.

Non-cryostable superconducting magnets can develop resistive zones (or“quenches”) in their interiors. If such a zone develops, high levels ofenergy, in particular current, in the magnet can cause the magnet severedamage. Systems therefore have been developed for dischargingsuperconducting magnets in an attempt to avoid such damage. Theforegoing is familiar to those skilled in the art of designing andfabricating superconducting magnets.

SUMMARY OF THE INVENTION

The invention is directed to discharging a superconducting magnet. Ingeneral, in one aspect, the invention features a method which detects aquench in the superconducting magnet, and which discharges thesuperconducting magnet into a load at a substantially constant voltagein response to detecting the quench.

Discharging the magnet at a substantially constant voltage decreasesdischarging time (relative to constant power or constant resistancedischarge). As a result, the likelihood (and/or amount) of damage to themagnet can be reduced. Also, if the magnet is connected to a utilitysystem, for example, through an inverter, discharging the magnet at asubstantially constant voltage increases the rate at which power can besupplied to the utility network. As a result, the utility network can bestabilized more quickly following a fault.

This aspect of the invention may include one or more of the followingfeatures. The voltage may be discharged through an inverter arrangedbetween the superconducting magnet and the load. An input of theinverter receives voltage from an output of the superconducting magnet,and the substantially constant voltage is maintained at the input of theinverter by controlling a phase relationship between voltage and currentat an output of the inverter. The inverter may be operated in overloadmode during constant voltage discharge. Operating the inverter inoverload mode further decreases magnet discharging time.

The load may comprise a utility network and/or one or more resistiveelements. One of the advantages of discharging the magnet into a utilitynetwork is that it reduces the need for additional circuitry fordischarging the magnet. The quench is detected by monitoring asuperconducting coil in the superconducting magnet. Discharginggenerally occurs until an amount of energy in the superconducting magnetis below a predetermined level. This reduces damage to the magnet. Themagnet may also be discharged into a load having a substantiallyconstant resistance. This may be done after the magnet has beendischarged at the substantially constant voltage, e.g., when the magnetis no longer in serious danger of damage.

Other advantages and features of the invention will become apparent fromthe following description and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 view of a superconducting magnet.

FIG. 2 is block diagram of circuitry for discharging the superconductingmagnet into a load.

FIG. 3 is a block diagram of a control board used in the circuitry FIG.2.

FIG. 4 is a flow diagram showing a process for discharging thesuperconducting magnet.

FIG. 5 is a flow diagram showing a process for controlling a phase angleof output current/power to discharge the superconducting magnet atconstant voltage.

FIG. 6 is block diagram of alternative circuitry for discharging thesuperconducting magnet into a load.

FIG. 7 is block diagram of alternative circuitry for discharging thesuperconducting magnet into a load.

FIG. 8 is block diagram of alternative circuitry for discharging thesuperconducting magnet into a load.

FIG. 9 is a vector diagram showing real and reactive components ofoutput AC power.

FIG. 10 is a flow diagram showing a process for discharging thesuperconducting magnet at constant voltage.

FIG. 11 is a block diagram of circuitry for compensating for inverterpower loss during discharging of the superconducting magnet.

FIG. 12 is a flow diagram showing a process for controlling the phaseangle of AC output current to keep the AC output voltage substantiallyconstant.

FIG. 13 is a graph showing a mode of magnet discharge.

FIG. 14 is a graph showing an alternative mode of magnet discharge.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, a superconducting magnet 10, representing asuperconducting magnetic energy storage (“SMES”) device and cryogenicrefrigeration unit, includes an energy storage magnetic coil 11positioned within a containment vessel 12 of the cryogenic refrigerationunit. In one embodiment, containment vessel 12 encloses magnetic coil 11in liquid helium 14, and is fabricated from two austenitic stainlesssteel vessels 13 separated by a vacuum insulated space 15. Inalternative embodiments, vessels 13 may be formed of stainless steel,aluminum, or epoxy fiberglass composite.

The cryogenic refrigeration unit includes one or more Gifford-McMahontype coolers (not shown) operating in concert to maintain cryogenictemperatures within vessel 12 and to re-liquify helium vapor building upwithin the vessel. Under normal operating conditions, helium (liquid orgaseous) does not circulate outside vessel 12. External, roomtemperature, refrigeration system gasses are not interchanged with theinternal helium supply. Preferably, the system design permits continuousoperation, with one or both coolers inoperable, for a minimum of 48hours.

Superconducting magnetic coil 11 is wound with a low-temperaturesuperconducting cable formed from niobium-titanium copper-based matrixwire that has been cabled into a mechanically stable form and insulatedprior to winding. An alternative embodiment uses a coil fabricated ofhigh-temperature superconductor. Superconducting coil 11 also can beembodied with a high temperature superconductor cooled with anappropriate cryogen, such as helium or nitrogen. A suitablesuperconducting magnetic coil is available from American SuperconductorCorporation of Westborough, Mass.

Discharging the Superconducting Magnet

FIG. 2 shows superconducting magnet 10 coupled to an AC (“AlternatingCurrent”) load 15. Load 15 may represent, but is not limited to, autility network, such as that described in U.S. patent application Ser.No. 09/240,751 and U.S. Provisional Application No. 60/117,784. Thus,load 15 may include reactive as well as resistive elements (not shown).

Arranged between load 15 and magnet 10 is circuitry 16 which couplesmagnet 10 to load 15. Circuitry 16 includes a magnet charger 19 forcharging magnet 10. Charger 19 may be an inverter or other device whichreceives power from the utility network or any other external source.Circuitry 16 also includes a DC-AC (with “DC” referring to “DirectCurrent”) power inverter 17; however, other types of circuitry may beused instead of, or in addition to, such an inverter. Inverter 17 can beany type of DC-AC power converter, such as single level H-bridge, singlelevel six-switch, multi-level, and stacked H-bridge inverters.

Inverter 17 operates in a “discharge” mode, during which energy isprovided from magnet 10 to load 15. In the “discharge” mode, inverter 17converts DC power from superconducting magnet 10 into AC power, andprovides that AC power to load 15. In this mode, thyristor 20, whichserves as a switch, is biased to produce an open circuit and diode 22,which also serves as a switch, is biased to produce a short circuit.This allows current to flow from magnet 10 to load 15.

Control circuitry 24 detects increases in resistance (i.e., quenches) insuperconducting magnet 10 and, in response, controls inverter 17 andswitch 20 so that Ad superconducting magnet 10 discharges its energyinto load 15 at a substantially constant voltage (V_(DC) acrossterminals 25). Control circuitry 24 may include, for example, a quenchcomparator circuit. The quench comparator circuit detects increases involtage across one or more portions of coil 11, which are indicative ofresistance in coil 11. Control circuitry 24, including the quenchcomparator circuit, may be implemented as hardware or as software(computer instructions) executing on one or more controllers.

For example, as shown in FIG. 3, control circuitry 24 may comprise threecontrollers 27, 29 and 30 (e.g., microprocessors) on a single circuitboard. In addition, this circuitry includes appropriate driver circuitsand analog-to-digital (“A/D”) converters (not shown).

Controller 27 executes computer instructions to control current andvoltage loops in inverter 17 (for use in discharging magnet 10 andmaintaining a constant output AC voltage from inverter 17, as describedbelow). Controller 29 functions as the system controller, meaning thatit executes computer instructions to detect an amount of power at load15, to detect the quench status of superconducting magnet 10, to open orclose switch 20, to control magnet charger 19, and to provideinformation to controller 27. Controller 30 records information obtainedby or generated by controllers 27 and 29, formats that information, anddisplays it to a user. Although this embodiment shows three controllers,any number of controllers (e.g., one controller) can be used to performthe functions attributed to controllers 27, 29 and 30. The computerinstructions executed in each controller may be stored in one or morememories (not shown) in circuitry 24 or an internal memory of eachcontroller.

FIG. 4 shows a process 32 for discharging magnet 10. Process 32 beginsby detecting 401 a quench in magnet 10. As noted, the quench is detectedwhen a change, in particular an increase, is detected in the voltageacross superconducting coil 11 in magnet 10. Detecting 401 may beperformed by a cryostat controller (not shown) or by controller 29. Thecontroller monitors the voltage across two or more symmetric windingsegments of magnet coil 11. If the voltage difference between thesymmetric segments exceeds a predetermined threshold, a quench isdetected. In response to the increase in voltage, controller 29instructs controller 27 to discharge energy from magnet 10. Controller27 controls discharging (402) so that V_(DC) remains substantiallyconstant throughout the discharging time.

To maintain V_(DC) substantially constant, controller 27 controls aphase relationship between the voltage and the current at the outputterminals 34 (FIG. 2) of inverter 17. Specifically, controller 27perturbates the nominal 90 degree phase angle between output AC voltageand output AC current to maintain the voltage level at DC terminals 25at a substantially constant value. Normally, to keep V_(DC) constant, asthe output current from magnet 10 (namely, I_(DEVICE)) decreases, theoutput current magnitude from inverter 17 would be decreased. Since theoutput power is the product of the output current and utility voltage,this effectively decreases the inverter's output power to match thepower available from magnet 10, which is the product of I_(DEVICE) andV_(DC). For utility grid stabilization, however, it is desirable tomaximize both real and reactive power. Because of this, it is preferableto operate the inverter at its maximum output current rating. Hence,controlling the phase angle of the inverter output current with respectto the utility voltage is the way by which this embodiment regulatespower flow and, thus, V_(DC).

One process for controlling the phase relationship between voltage andcurrent is shown in FIG. 5 below; however, other processes may be used.During discharge, inverter 17 may be operated in overload mode (e.g.,substantially above its continuous power rating). The period duringwhich the inverter can operate in overload mode depends upon the thermalcapacity of the inverter.

Returning to FIG. 4, once magnet 10 is discharged to a predeterminedlevel, process 32 ends. During normal discharge of magnet 10, forutility grid voltage stabilization purposes, magnet 10 may be dischargedto a non-zero value. However, when magnet 10 is discharged in responseto a quench, magnet 10 may discharge to zero.

Other circuit configurations may also be used to implement magnetdischarging process 32. For example, FIG. 6 shows magnet 10 coupled to aDC load 36. In this configuration, DC load 36 includes two gate turn-offdevices, such as IGCT (“Integrated Gate Commutated Thyristor”) powerswitching devices 37 and 39 which are connected to resistive load 39 a.Additional capacitive-inductive-resistive circuitry (e.g., a snubbercircuit) (not shown) may be connected in parallel to the gate turn-offdevices. IGCT devices 37 and 39 are alternatively opened and closed (bycontrol circuitry 24) to discharge magnet 10 in such a way that asubstantially constant voltage V_(DC) is maintained across terminals 25.The resistive load 39 a and IGCT devices modulate the discharged powerin accordance with control signals from control circuitry 24. Outputdiodes 38 a and 38 b control power flow between IGCT devices 37 and 39.IGCT devices 37 and 39 may also be other gate-controlled semiconductorswitches, such as, but not limited to, GTO's (“Gate Turn-offThyristors”) or IGBT's (“Insulated Gate Bipolar Transistors”) usedeither singly or in series/parallel.

FIG. 7 shows still another circuit configuration that may be used toimplement magnet discharging process 32. This configuration is similarto that of FIG. 2, except that it includes a profiling circuit 40 inshunt across terminals 41. Profiling circuit 40 includes a resistor 42and an SCR (“Silicon Controlled Rectifier”) device 44 which operates aseither an open or closed switch. If the current of magnet 10 drops belowa predetermined level, such as below 25% of its maximum value (whichusually occurs near the end of a discharging cycle), controller 27outputs a signal to close the switch of SCR device 44. Triggering SCRdevice 44 causes current from magnet 10 to flow through profilingcircuit 40, thus changing the discharging process from a constantvoltage discharging process to a constant resistance dischargingprocess.

A relatively small resistance for resistor 42 causes magnet 10 to do aresistive discharge, in which the voltage of magnet 10 is low anddischarge time is long (see FIG. 13). A relatively higher value forresistor 42 results in a constant voltage discharge for times t₀-t₁ andin a constant resistance discharge after time t₁ (see FIG. 14).

Profiling circuit 40 can be “switched in” at other points during thedischarging process. For example, if inverter 17 malfunctions before orduring magnet discharging, profiling circuit 40 can be “switched in” todischarge magnet 10.

Controlling Power Phase Angle to Maintain Constant Voltage

Referring to FIG. 8, the real power output from inverter 17 is a productof I_(DEVICE) and V_(DC) where I_(DEVICE) is the DC current from magnet10 and V_(DC) is the input DC voltage to inverter 17. As magnet 10drains, the value of I_(DEVICE) decreases. To keep V_(DC) substantiallyconstant in spite of this decrease in I_(DEVICE), inverter 17 varies thephase angle of its output AC current (and thus the phase angle of theoutput AC power). Inverter 17 changes the phase angle to increase powerflow towards the DC side or into the inverter when V_(DC) is below anominal value and to increase power flow out of the inverter when V_(DC)is above the nominal value.

Referring to FIG. 9, a vector diagram 50 illustrates the (sinusoidal) ACpower output by inverter 17 as a function of real power (axis 52) andreactive power (axis 54). At real power axis 52, I_(DEVICE) is at itsmaximum value. This corresponds to a time when superconducting magnet 10is fully, or almost fully, charged. Inverter 17 outputs its maximumamount of real power (“kW”) along real power axis 52. This is becausereal power is a function of the product of V_(DC) and I_(DEVICE) andI_(DEVICE) is at its maximum. The vector output would lie on the realpower axis 52 only for the special case of magnet power(V_(DC)*I_(DEVICE)) equal to the maximum inverter rating. Typically, theinverter power rating is greater than magnet power.

Along the reactive power axis 54, I_(DEVICE) is at its minimum (in thiscase, zero). Inverter 17 outputs its maximum amount of reactive power(“kVAR”) along reactive power axis 54 and uses the energy from thisreactive power to maintain V_(DC) substantially constant (since reactivepower does not result in aggregate energy dissipation to the load).Reactive power may be sourced from the utility network and then outputback to the utility network, or it may be obtained from other sources.

Between axes 52 and 54, the power output of inverter 17 is a combinationof both real and reactive components. Power vector 51 represents theamount of power that is output by inverter 17. The degree to which powervector 51 is real or reactive is determined on the basis of phase angle(“θ”) 55 between vector 51 and real power axis 52. For example, todetermine the amount of real power provided by vector 51 at θ, a line 56is drawn from vector 51 to real power axis 52. Intersection point 59corresponds to the amount of real power provided by vector 51. Reactivepower is determined similarly (by drawing a line from vector 51 toreactive power axis 54).

As energy is discharged from magnet 10, I_(DEVICE) drops. The amount ofcurrent (and thus real power) output from inverter 17 can be reducedcorrespondingly by varying the phase angle. For example, in FIG. 8, thephase angle of the output current can be varied so that less real poweris provided by inverter 17 (thus increasing θ).

FIG. 5 shows a process 62 for varying the phase angle of the AC outputcurrent during magnet discharging to keep V_(DC) constant. Process 62can be implemented as a computer program (instructions) executed bycontroller 27. Controller 27 first obtains a value of V_(DC) (501). Thisvalue may be obtained, e.g., from controller 29 (which, as describedabove, monitors V_(DC)). Alternatively, controller 27 may obtain thevalue of V_(DC) itself (by monitoring terminals 41) or from anothersource.

The phase angle of the AC power output from inverter 17 is controlled tokeep V_(DC) substantially constant (502). Controlling is performed bycomparing (503 to 506) the value for V_(DC) obtained in step 501 to atarget (predetermined) value. This value may be preset in controller 27,or it may be set, for example, based on an initial value of V_(DC).Alternatively, a controller within inverter 17 could perform thisfunction.

The results of the comparisons in steps 503 to 506 determine how much,and in what direction, to vary the phase angle of the inverter's outputAC current. For example, if process 62 determines (503) that themeasured value of V_(DC) is considerably larger than the target value(for example, a voltage nearing the inverter's maximum voltage rating),then process 62 decreases (507) the phase angle at a fairly rapid rate,such as the inverter's slew rate limit. This rapidly increases theamount of real power being supplied to utility network 61, andcorrespondingly decreases the value of V_(DC). If process 62 determines(504) that the measured value of V_(DC) is larger than the target value,but not inordinately so, then process 62 decreases (508) the phase angleat a less rapid rate, such as half of the inverter's slew rate limit.This increases the amount of real power being supplied to utilitynetwork 61, and correspondingly decreases the value of V_(DC).

On the other hand, if process 62 determines (505) that the measuredvalue of V_(DC) is considerably less than the target value, then process62 increases (509) the phase angle at a fairly rapid rate, such as theinverter's slew rate limit. This rapidly decreases the amount of realpower being supplied to utility network 61, increases the amount ofreactive power being supplied, and correspondingly increases the valueof V_(DC). If process 62 determines (506) that the measured value ofV_(DC) is less than the target value, but not inordinately so, thenprocess 62 increases (510) the phase angle at a less rapid rate, such ashalf of the inverter's slew rate limit. This decreases the amount ofreal power being supplied to utility network 61, increases the amount ofreactive power being supplied, and correspondingly increases the valueof V_(DC) Process 502 depicts a proportional control system which can beimplemented in hardware, software, or a combination of the two. Insimple terms, an error signal (V_(DC)−V_(TARGET)) is used to control asystem variable (phase angle). The rate-of-change of the system variableis proportional to the error, hence the name: proportional controller.

The foregoing comparisons 503 to 506 operate to maintain V_(DC)substantially constant during discharging (or charging) of magnet 10.While four such comparisons are shown, the invention is not limited assuch. For example, there may only be two comparisons—one for determiningif V_(DC) is greater than the nominal value and one for determining ifV_(DC) is less than the nominal value. In this case, only one rate isused for increasing the phase angle of the output AC current and onerate is for decreasing that phase angle. Alternatively, the inventionmay include more than the four comparisons of process 62, each with itsown corresponding rate for varying the phase angle.

Process 62 may be incorporated into other processes. For example, FIG.10 shows a process 64 that uses process 62 to discharge superconductingmagnet 10 (FIG. 8) at a substantially constant V_(DC) voltage. Process64 may be implemented by a computer program (instructions) executing onone or more of controllers 27, 29 and 30 (FIG. 3).

To begin, process 64 monitors (1001) magnet 10 to detect a quench. Thisis described above. Once a quench has been detected, process 64 sets(1002) the phase angle of the inverter's output AC current to 90° (fullleading). Process 64 initiates (1003) the overload mode of inverter 17,and opens (1004) switch 60 (FIG. 8) thereby allowing all current frommagnet 10 to flow to inverter 17. In addition, it commands magnetcharger 19 to turn off, thereby preventing additional power from flowingto the magnet. A minimum overload value in this case is 100% overloadfor one second (or two PU for one second, where “PU” is “Per Unit”, theideal continuous output current of inverter 17). Process 64 moves (1005)the phase angle of the output current in accordance with detectedconditions in the magnet. For example, if a large resistive zone isdetected in magnet 10, process 64 may decrease the phase angleconsiderably to discharge the magnet quickly. Then, process 64 calls(1006) process 62 for discharging the magnet while maintaining V_(DC)substantially constant. Process 62 is performed until (1007) magnet 10reaches a preset energy level, which is generally not zero.

Process 64 closes (1008) switch 60 and determines (1009) if the overloadmode of the inverter has ended. Once the overload mode ends, process 64ends, where after magnet 10 may be repaired and/or replaced, asrequired.

Power losses in inverter 17 may result in minor deviations in the phaseangle of the output power. Such losses can cause minor fluctuations inV_(DC) during charging and/or discharging. FIG. 11 is a block diagram ofcircuitry 70 that can be used to compensate for the power losses ininverter 17, and thus reduce fluctuations in V_(DC). This circuitry 70can be implemented using discrete hardware components and/orinstructions executing on a controller. Master controller may be, forexample, the control circuitry 24 of FIG. 3. PLL 74 may be implementedin software within one of controllers 27, 29 or 30. Phase compensator 71may reside in software within a controller in inverter 17.

Circuitry 70 includes phase angle compensator 71, master controller 72,and phase-locked loop (“!PLL”) 74. Also included in FIG. 11 are monitor75 and DC bus capacitor 76. DC bus capacitor 76 stores an actual valueof V_(DC) across the input terminals of inverter 17. Monitor 75 monitorsthis value, compares it to a preset target voltage value, and determinesa difference between the actual value of V_(DC) and the preset nominalvalue. This information is provided to phase angle compensator 71.

PLL 74 and master controller 72 determine a phase angle of power onutility system 61 and output current sampled from inverter 17. Thisdetermination is based on voltage sampled from utility system 61. Thephase angle is provided to phase angle compensator 71 (via mastercontroller 72). Phase angle compensator 71 uses this phase angle and thedifference in voltage provided from monitor 75 to determine a phaseangle offset. The phase angle offset is the amount by which a phaseangle set by inverter 17 must be offset to compensate for power lossesin inverter 17. Generally, the phase angle offset has a negative value,which means that inverter 17 receives power from utility system 61 inorder to compensate for the lost power.

Maintaining Constant Output Voltage

Inverter 17 is also controlled to maintain a substantially constantoutput AC voltage. The value of this output voltage may be dictated bythe utility network or by whatever system is receiving power frominverter 17.

The output AC voltage is controlled by regulating the output AC currentand phase from inverter 17. Specifically, controller 27 senses (FIG. 3)the output AC voltage at terminals 77 of inverter 17 (FIG. 8) andregulates the magnitude of reactive current output from inverter 17 inorder to keep the output AC voltage substantially constant. FIG. 12shows a process 80, which is implemented by computer instructionsexecuting on controller 27, for maintaining a substantially constantoutput AC voltage from inverter 17.

Process 80 begins by integrating (1101) the root-mean-square (“RMS”) ofthe AC voltage output from inverter 17 over a period of time. Thisintegration is performed several times on all three phases of voltage,and the results from all three phases are summed and are averaged (1102)to determine the average output voltage V_(avg) of inverter 17. Process80 compares (1103) V_(avg) to a target voltage value, typically 1 PUwhich, in this example, is 480 V. If V_(avg) is greater than thisnominal value, process 80 decreases (1104) the reactive current out ofinverter 17, which causes a corresponding decrease in output AC voltage.If V_(avg) is less than this nominal value (1105), process 80 increases(1106) the reactive current out of inverter 17, which causes acorresponding increase in output AC voltage. Process 80 may beimplemented independently on each phase or line of the utility systemusing a 3-phase inverter of a suitable type or three single-phaseinverters.

It is noted that the reactive current can be leading or lagging. Toboost or increase the voltage on a utility system, one can eitherdecrease the amount of lagging reactive current or increase the amountof leading reactive current being injected into the utility. Likewise,to buck or decrease the utility voltage, one can decrease the leadingcurrent or increase the lagging injected current.

Other embodiments not described herein are also within the scope of thefollowing claims. For example, the invention can be used in connectionwith any current-mode energy storage device, such as a synchronousflywheel, and not just superconducting magnets. Also, combinations ofhardware and/or software not described herein may be used. For example,GTO (“Gate Turn-off Thyristor”) or IGBT (“Insulated Gate BipolarTransistor”) switches may be used in the embodiments of the foregoingfigures. The invention may be used in a DSMES (“DistributedSuperconducting Magnetic Energy Storage System”), such as that describedin U.S. patent application Ser. No. 09/240,751 and U.S. ProvisionalApplication No. 60/117,784.

What is claimed is:
 1. A method of discharging a superconducting magnet,comprising: detecting a quench in the superconducting magnet; anddischarging the superconducting magnet into a load at a substantiallyconstant voltage in response to detecting the quench.
 2. The method ofclaim 1, wherein the voltage is discharged through an inverter arrangedbetween the superconducting magnet and the load.
 3. The method of claim2, wherein: an input of the inverter receives voltage from an output ofthe superconducting magnet; and the substantially constant voltage ismaintained at the input of the inverter by controlling a phaserelationship between voltage and current at an output of the inverter.4. The method of claim 3, wherein the inverter is operated in overloadmode during discharge of the voltage from the superconducting magnet tothe load.
 5. The method of claim 1, wherein the load comprises a utilitynetwork.
 6. The method of claim 1, wherein the load comprises one ormore resistive elements.
 7. The method of claim 1, wherein the quench isdetected by monitoring a superconducting coil in the superconductingmagnet.
 8. The method of claim 1, wherein discharging occurs until anamount of energy in the superconducting magnet is below a predeterminedlevel.
 9. The method of claim 1, further comprising discharging thesuperconducting magnet into a load having a substantially constantresistance.
 10. The method of claim 9, wherein discharging thesuperconducting magnet into the load having the substantially constantresistance occurs after discharging the superconducting magnet at thesubstantially constant voltage.
 11. A system for discharging asuperconducting magnet, comprising: a load; and circuitry which (i)detects a quench in the superconducting magnet, and (ii) discharges thesuperconducting magnet into the load at a substantially constant voltagein response to detecting the quench.
 12. The system of claim 11, whereinthe circuitry comprises an inverter arranged between the superconductingmagnet and the load.
 13. The system of claim 12, wherein: the inverterincludes an input from the superconducting magnet and an output to theload, the input for receiving voltage from an output of thesuperconducting magnet, and the output for discharging AC power to theload; and the inverter maintains the substantially constant voltage atthe input of the superconducting magnet by controlling a phaserelationship between voltage and current in the AC power at the outputof the inverter.
 14. The system of claim 12, wherein the inverteroperates in overload mode during discharge of the voltage from thesuperconducting magnet to the load.
 15. The system of claim 11, whereinthe load comprises a utility network.
 16. The system of claim 11,wherein the load comprises: one or more resistive elements; and switcheswhich control connection of the one or more resistive elements to thesuperconducting load to maintain discharge at the substantially constantvoltage.
 17. The system of claim 11, wherein the quench is detected bymonitoring a superconducting coil in the superconducting magnet.
 18. Thesystem of claim 11, wherein discharge occurs until an amount of energyin the superconducting magnet is substantially dissipated.
 19. Thesystem of claim 11, further comprising a profiling circuit coupled inshunt between the superconducting magnet and the load, the profilingcircuit comprising a switch and one or more resistive elements; whereinthe switch connects the profiling circuit to the output terminals of thesuperconducting magnet after a predetermined amount of energy has beendischarged from the superconducting magnet.